The invention relates to an optical parametric waveform synthesizer being configured for creating synthesized optical waveforms and to a method for synthesizing optical waveforms. Applications of the invention are available in particular in high-energy short-pulse laser sources based on OP(CP)A technology, in particular in OP(CP)A based waveform synthesizers (“Third Generation Femtosecond Laser Sources”).
In the present specification, reference is made to the following publications cited for illustrating prior art techniques.    [1] S.-W. Huang et al. in “Nat. Phot.” (vol. 5, no. 8, 2011);    [2] C. Manzoni et al. in “Laser & Photonics Reviews” (vol. 9, no. 2, 2015);    [3] O. D. Muecke et al. in “IEEE JSTQE” (vol. 21, No. 5, 2015);    [4] T. R. Schibli et al. in “Optics Letters” (vol. 28, no. 11, pp. 947-949 (2003));    [5] J. Ye et al. in “Physical Review Letters” (vol. 87, no. 27, pp. 270801-1-270801-4 (2001));    [6] K. W. Holman et al. in “Optics Letters” (vol. 28, no. 23, pp. 2405-2407 (2003));    [7] S. T. Cundiff et al. in “Review of Scientific Instruments” (vol. 72, no. 10, pp. 3749-3770 (2001)); and    [8] K. Numata et al. in “Applied Physics B: Lasers and Optics” (vol. 116, no. 4, pp. 959-966, 2014).
The demand for ultrashort and ultra-broadband laser-pulses is increasing. Optical parametric chirped-pulse amplification (OP(CP)A) is the most promising candidate for a scalable high-energy, high-average power ultrashort pulse source, allowing the creation of multi-octave spanning spectra with controlled phase behavior, energy in the mJ range and at high repetition rate. In particular, the OP(CP)A technique allows for transferring energy from laser pulses with durations ranging from tens of femtoseconds to picoseconds and even nanoseconds (pump pulses) into pulses sustaining ultrashort compressed durations of few femtoseconds signal pulses. OP(CP)A amplifier crystals experience only low thermal load and high-energy pump lasers are available (e.g. Yb:YAG, Yb:YLF, Nd:YAG).
Based on the OP(CP)A technique, optical waveforms can be synthesized, as described e.g. in [1], [2] and [3]. An optical parametric waveform synthesizer typically includes multiple OP(CP)A channels with different spectral characteristics, which are coherently combined for synthesizing pulses (optical waveforms) with a desired temporal shape and spectral content. The optical parametric waveform synthesizer requires a well synchronized overlap in time and phase of the pump and seed pulses in the OP(CP)A channels and of the output pulses of the OP(CP)A channels.
With more details, from a common pump laser source or different synchronized pump laser sources, a passively-CEP-stable white-light seed is generated, split into different spectral OP(CP)A channels and stretched to match the pulse duration of the pump source(s). Each channel includes a chain of OP(CP)A-stages to reach the desired energy level. After amplification the pulses are recombined, e.g., with dichroic beam splitters for creating the optical waveforms to be obtained. Another possibility is to make use of different white-light seeds each optimized for different spectral OP(CP)A channels, all ideally derived from a common CEP-stable pulse.
To be able to synthesize/shape the light-field of the optical waveforms one needs to stabilize and control the envelopes and phases of the pulse(s) between the outputs of the different OP(CP)A channels as well as the carrier-envelope phases (CEPs) of the pulses. Furthermore, the pump and seed pulses need to be synchronized at each OPA stage.
The CEP of the white-light source might be passively stabilized (by passive-CEP stabilization e.g. DFG via OPA) and/or actively stabilized by measuring the f-2f beating and using a feedback control onto an appropriate optical element (e.g., changing dispersion by increasing/decreasing of glass wedges insertion), in order to account for environmental changes. Each channel of the seed is offered to an OPA-stage while ensuring temporal overlap with the corresponding pump pulse. This overlap does not need to be actively synchronized if the passive timing drifts between pump and seed are negligible compared to the duration of the pulses.
The arrival time difference (envelope time delay) between the outputs of two OP(CP)A channels can be measured by an appropriate technique, e.g., RAM, balanced optical cross-correlator (BOC), see e.g., [4], and the measured arrival time difference can be used to feedback control a delay line on the beam path of the pump of the last OP(CP)A stage of one of the two OP(CP)A channels. This locks together the pulse envelopes of the output of two OP(CP)A channels.
The other critical temporal property, in order to achieve a shot-to-shot stable synthesis, is the phase between the carrier waves of the different OP(CP)A channels. These are mainly affected by the interferometric instability between the different OP(CP)A channels from the splitting of the common seed (or common driver of the seeds) to its recombination after amplification. The relative phase between the carrier waves can be measured by beating spectral components from two separate outputs (if necessary by extending the spectral range of one or both outputs by means of nonlinear effects). The observed fringes in the spectrum carry information about the relative phase between the two contributing channels. This might take place in a high-resolution and single-shot spectrometer with dedicated electronics (e.g. FPGA, DSP, CPU) to calculate and track the phase of the fringe pattern, which allows for low-latency control, in particular active stabilization.
Accordingly, controlling the optical parametric waveform synthesizer, in particular stabilizing CEP and controlling the envelopes and phases between the outputs of the different OP(CP)A channels requires multiple control loops. However, the control loops are inherently coupled and influence each other, as exemplified in the following.
In the optical parametric waveform synthesizer, first the spectral modulations can be observed in the spectral overlap regions between adjacent channels to retrieve the relative phase between the OP(CP)A channel outputs. If this quantity needs correction, this is performed by moving a delay stage on the seed or amplified seed beam path before the last amplifier stage of one of the channels. This action ensures to lock the relative phase between the channels. However this produces a small change in the arrival time of the OP(CP)A channel output at the synthesis point. Recognizing this drift and moving the pump at this subsequent OPA stage requires time and results in a shift in the CEP of the amplified pulse. This mutual influence of the control loops requires a long time to stabilize and maybe never reach stable operation because each control loop will partly counteract the side effects of the other to bring its quantity back to the set-point.
Accordingly, control of conventional parametric waveform synthesizers is challenging (see e.g., [2]). In particular, it includes actuators with long response time which limits the bandwidth for control and stabilization, resulting in substantial limitations in practical applications of parametric waveform synthesizers.
It is generally known that inherent mutual interactions in coupled control loops can be decoupled by orthogonalization of the control, wherein the control is performed in such a way, that the signals given to the actuators are correlated such that only one controlled quantity, e.g., timing or phase, is varied at a time. Orthogonalized control has been mentioned in [2], [5], [6] and [7] controlling optical frequency synthesis with a mode-locked laser. However, this scheme was restricted to synchronizing higher repetition rate laser sources for example by controlling a mirror of the mode-locked laser, and it cannot be applied in controlling optical parametric waveform synthesizers. It was assumed in [2] that high-speed fluctuations in carrier phase and pulse timing between spectral channels would be common mode. Accordingly, [2] even suggests, that a specific control of these properties in optical parametric waveform synthesizers would not be necessary.
Publication [8] disclosing fast-switching a methane lidar transmitter based on a seeded optical parametric oscillator represents technological background with regard to OPA applications. However, publication [8] does not refer to the control of mode-locked lasers or the application of femtosecond or picosecond pulses.
The objective of the invention is to provide an improved optical parametric waveform synthesizer and an improved method of synthesizing optical waveforms, being capable of avoiding disadvantages and limitations of conventional techniques. In particular, the optical parametric waveform synthesizer is to be capable of synthesizing optical waveforms with a complete control on phase and timing from each spectral channel, in order to achieve huge tunability of the waveform, together with improved stability at shortened control response time.
These objectives are solved with an optical parametric waveform synthesizer and a method for synthesizing optical waveforms of the invention.